1. Field of the Invention
The invention relates to a proton exchange membrane composition, and more particularly to a proton exchange membrane composition for high temperature conductivity.
2. Description of the Related Art
Fuel cells are well known and are commonly used to produce electrical energy by means of electrochemical reactions. Compared to conventional power generation apparatuses, fuel cells have advantages of causing less pollution, generating less noise, increased energy density and higher energy conversion efficiency. Fuel cells can be used in portable electronic products, home-use or plant-use power generation systems, transportation vehicles, military equipment, space industry application, large-sized power generation systems, etc.
For example, in the case of a proton exchange membrane fuel cell (PEMFC), hydrogen is supplied to an anode and an oxidation reaction occurs in the presence of an anode catalyst layer, thus protons and electrons are generated. The protons reach the cathode through the proton exchange membrane. Meanwhile, in the cathode, electrons from the anode via the external circuit are reduced to oxygen supplied to the cathode and protons by reduction, producing water.
FIG. 1A shows an exploded view of conventional fuel cell 10 with a membrane electrode assembly, and FIG. 1B shows a cross-section view of FIG. 1A. As shown in FIGS. 1A and 1B, the conventional fuel cell 10 can comprise a membrane electrode assembly 12 comprising a catalytic anode film 121, a proton exchange membrane 122, and a catalytic cathode film 123, wherein a binder composition 124 can be used to combine the catalytic anode film 121 and the proton exchange membrane 122, and/or the catalytic cathode film 123 and the proton exchange membrane 122. The conventional fuel cell 10 further comprises a bipolar plate 13 and two end electrode plates 11 for connection, wherein the bipolar plate 13 and the end electrode plates 11 comprises gas passages 111 and 131 for conducting hydrogen and oxygen into the membrane electrode assembly 12.
In general, conventional proton exchange membrane fuel cells (PEMFCs) include a Nafion-based proton exchange membrane. Since Nafion only exhibits acceptable electrical conductivity with high water content, the Nafion-based proton exchange membrane has an operating temperature of below 90° C. (70˜80° C. in general).
In low operating temperatures however, proton exchange membrane fuel cells, have two key problems. First, platinum catalyst is apt to be reacted with minute amounts of CO existing in hydrogen gas, resulting in inferior catalytic efficiency. Second, water management is difficult to control. Inefficient water management may lead to the anode becoming prone to drying and the cathode to flooding, resulting in oxygen not being able to contact the surface of the catalyst, thus limiting proton transport.
Proton conduction in proton exchange membranes is achieved by either the vehicular or Grotthuss mechanism.
In the vehicular mechanism, protons transfer through the proton exchange membrane together with water molecules (H2O) to form hydronium ion (H3O+). Therefore, proton conductivity depends on the water retention ability of the proton exchange membrane. However, water molecules are apt to scatter at high temperatures. The proton exchange membranes including materials with hydrogen sulfate groups (such as Nafion) transfer proton based on the vehicular mechanism.
In the Grotthuss mechanism, the hydrogen ions (protons) traverse the proton exchange membrane by hopping from different proton acceptor sites in the absence of water. In general, proton exchange membranes, based on the Grotthuss mechanism include Brönsted acid base pairs (ionic liquids) or are doped with excessive protonic acid. The proton conductivity and the operating temperature in the Grotthuss mechanism is a direct ratio (especially for temperatures higher than 130° C.). The polybenzimidazole proton exchange membranes have been an exponent of the proton exchange membranes based on the Grotthuss mechanism. However, the electrical conductivity of polybenzimidazole measured at 160° C. is less than that of the Nafion measured at 80° C.
Overall, to achieve high temperature proton conduction for proton exchange membranes, water retention ability is increased, chemical resistance is increased, flexibility is increased, and/or mechanical strength of the proton exchange membrane during high temperature operation is increased.
A proton exchange membrane, having polybenzimidazole (PBI) as a main component, doped with phosphoric acid or sulfuric acid, has been disclosed. The operating temperature of the PBI based proton exchange membrane can be 150˜200° C., and the CO tolerance of the proton exchange membrane fuel cells can be enhanced to 1% at 160° C. The PBI-based proton exchange membrane, however, has an ion conductivity of 1 mS/cm (measured at 120° C.), lower than that of immersed Nafion-based proton exchange membrane (60 mS/cm measured at 80° C.). Further, the power density of PBI-based proton exchange membrane is also less than that of the Nafion-based proton exchange membrane.
Accordingly, a novel proton exchange membrane for membrane electrode assemblies for replacing the conventional Nafion-based proton exchange membrane is required.